Various devices have been used for measuring the rate of flow of the liquid in a tube or pipe. Hereafter the term “tube” is used to include any type of conduit in which a liquid can flow that can be used with the prior art and the invention.
For example, a variety of types of flow measuring devices used in chemical/pharmaceutical industries exist in which contact is made with the liquid as the flow rate measurement is being made. Such devices include a Coriolis flow meter which measures mass flow as a function of gyroscopic torque forces. Devices using this method are complex and expensive. They also have an accuracy of about ±0.4%. Another device is an ultrasonic flow meter which is suited for measuring gallons-per minute flow and whose accuracy is ±0.5%. There also is a continuous heat addition flow meter in which the liquid is heated by a probe immersed in the liquid and the downstream temperature is continuously measured, such as by a thermistor type sensor. In this type of device the accuracy of the measurement varies with the specific heat of the metered liquid and with ambient temperature fluctuations. Also available is a self-heating thermistor placed in contact with the liquid. The thermistor undergoes cooling proportional to the rate of flow of the liquid flowing past it. This type of device is nonlinear and the accuracy of the measurement result varies with the specific heat of the liquid and ambient temperature variations. U.S. Pat. Nos. 5,726,357 and 5,623,097 each disclose a semiconductor substrate on which is integrated a heating element and a heat sensing element. The fluid passes over the heating element and is detected as it passes over the sensing element.
In many applications it is desirable, and even necessary, to measure the liquid flow rate non-invasively, that is, without any part of the measuring device coming into contact with the liquid. This preserves the sterility of the liquid. Applications that require non-invasive measurement include medical devices such as infusion pumps for drug delivery, devices that feed nutrients to patients, and applications in which a disposable tube is used such as in a drug delivery system.
In many applications in which the flow rate is to be measured non-invasively the liquid flow rate is relatively low. Existing devices have difficulty in providing accurate measurement for low flow rate applications. Accurate measurement of low volumetric liquid flow rate is very important in analytical chemistry applications such as chromatography and capillary electrophoresis.
A number of systems exist for measuring liquid flow rate non-invasively. Typical of these is the system described in U.S. Pat. No. 5,764,539, in which a non-invasive temperature sensor is heated to a predetermined temperature which changes as the liquid passes by it. The temperature change is determined to detect the characteristics of the liquid and whether or not the liquid is flowing. In U.S. Pat. No. 4,938,079 a resonant microwave cavity provides heat markers in the flowing liquid which are detected by another resonant cavity based on the perturbations of the liquid by the heat markers.
In U.S. Pat. No. 6,582,393 an amount of liquid to be used as a medicinal dose is held in a chamber in an elastic tube formed by a pinch bar engaging the tube. The dose amount of liquid is heated by a heating block and is then released by releasing the pinch bar. The heated liquid dose is sensed by a heat sensor block and the travel time of the dose between the heating and sensing blocks is computed to give the dose flow rate. This information is used to maintain or correct the time of application of further doses of the liquid to achieve a predetermined dose rate.
In U.S. Pat. Nos. 6,932,796 and 7,268,859 and U.S. patent publication 2005/0005710 a tube a heating element heats liquid flowing in a tube to form a heat marker that is optically detected. The travel time of the heat maker between the heating element and the optical detector is used to compute the flow rate. The optical detectors used in the systems of these patents and patent publication do not actually determine the temperature of the heat marker and the configuration of the optical detector is relatively complex.
All of the existing non-invasive liquid flow rate measure devices and systems are relatively complex and relatively expensive. Accordingly, a need exists to provide a system and method that can measure flow rates non-invasively, with such system being easy to operate, providing accurate results even for low volume flow rates and being of a relatively low cost.